Production of semiconductor material

ABSTRACT

When producing semiconductor bodies by a process of depositing semiconductor material from a gaseous mixture on an elongated crystalline semiconductor starting filament held between two laterally fixed supports, lateral stresses in the filaments are removed by allowing substantially friction-free, lateral movement of the filament as it is heated prior to the vapor deposition. This movement is allowed by melting a coupling segment between the filament and at least one of the supports to thereby allow lateral deformation of the coupling segment as any lateral stress of the filament is relieved. When this method is coupled with conventional means of relieving longitudinal stress in the filament, perfection in grown semiconductor bodies is maintained.

[ 51 Mar. 7, 1972 United States Patent Dyer Primary Examiner-William L. Jarvis [541 PRODUCTION OF SEMICONDUCTOR MATERIAL [72] Inventor:

Attorney-Samuel M. Mims, .lr., James 0. Dixon, l-lassell,

Lawrence D. Dyer, Richardson, Tex.

ABSTRACT Assignee: Texas Instruments Incorporated, Dallas,

Tex.

When producing semiconductor bodies by a proces of positing semiconductor material from a gaseous mixture on gated crystalline semiconductor starting filamen between two laterally fixed supports, ments are removed by allowing substantially friction-free, lateral movement of the filament as it is heated prior to vapor deposition. This movem coupling segment'between the fi supports to thereby allow lateral d ment as any lateral stress of the this method is coupled with convention longitudinal stress in the fila n b e 0: en da m 1 5 %5 19H19 ZM ZM 8 8 3H03H 2 2 A MA 6 6 0 0 l 1 7 7v 1 H 0 .R 1 7 .M m WM .0 W mm C m s m mm c U Q N "m d a n "n R v u 0 MN m "In N 8 m mm m m n w m m d H D. S. MM F A U IF 1 l. .1 ll. 1 2 .l 2 8 6 2 2 5 5 5 l .l i. .ll.

ment, perfection in grown UNITED STATES PATENTS semiconductor bodies is maintained.

Reuschel 117/106 PATENTEDW 7 I972 FIG! INVENTOR LAWRENCE D. DYER PRODUCTION OF SEMICONDUCTOR MATERIAL This invention relates to the production of semiconductor bodies. In another aspect, this invention relates to a method and apparatus for controlling the stress on a seed filament of semiconductor material during operations involving the vapor phase deposition of semiconductor material upon the filament.

Semiconductor materials such as silicon and germanium are commonly used in the manufacture of semiconducting devices such as diodes, transistors, and integrated circuits. A conventional method for producing the semiconductor material suitable for the manufacture of electronic components involves the vapor phase deposition of the semiconductor material on a heated filament made from the same material. According to this method an elongated generally cylindrical or flat-sided filament is placed within a reactor such as a quartz tube which is fitted with suitable end plates and graphite electrodes within which the ends of the starting filament are clamped. The filament is then heated by developing a potential across the graphite electrodes and thereby passing current therethrough.

During most operations the filament is initially heated at elevated temperatures and treated with vapors such as hydrogen and/or hydrogen halide to precondition the surface of the filament by etching. Next, the temperature of the filament is lowered somewhat, and a gaseous stream of hydrogen and a silicon halide is passed over the filament. The gaseous components in the stream react upon contacting the hot starting filament and thereby deposit silicon on the surface of the filament. Conventional procedures of this type are disclosed in U.S. Pat. Nos. 3,168,422 and 3,172,791..

When practicing the above-described process in conventional equipment, problems have arisen which result in nonuniform growth and/or defects in the grown semiconductor material due to (1) stresses which arise from sideward restraint and nonaxial positioning of the upper and lower chucks, and (2) stresses which occur due to longitudinal expansion of the filament as it is heated from ambient temperature to the etching and deposition temperature. These stresses can lead to bowing of the formed semiconductor rod and to irregularly deposited semiconductor rods. This in turn results in expensive handling problems during technological processing of the semiconductor rod. Additionally, if the filament is monocrystalline and it is desired to deposit semiconductor material in single crystal form, then the expansion can cause a stress increase that generates an undesirable dislocation content in the crystal which many times yields a product unsuited for electronic purposes.

Recently improved equipment has been devised for alleviating the stresses induced by longitudinal expansion of the filament during the deposition process. These devices are disclosed in my copending applications Ser. No. 744,028 (TI 2904) filed July 11, 1968 and Ser. No. 783,376 (TI 3364) filed Dec. 12, 1968. However, no effective method was heretofore known for alleviating the lateral stresses which are induced in the filament from sideward restraint and nonaxial positioning ofthe upper and lower electrode chucks within the vapor phase reaction chamber.

Therefore, one object of this invention is to provide an improved method for producing semiconductor bodies.

Another object of this invention is to provide a method of alleviating lateral stresses in a seed filament retained between electric leads in a reaction zone during a vapor phase deposition process for producing semiconductor rods.

Still a further object of this invention is to provide an improved process for alleviating both lateral and longitudinal stresses in a semiconductor filament as it expands and contracts between electrical leads during the vapor phase deposition process for producing semiconductor rods.

According to the invention, an improved method and apparatus is provided for growing a semiconductor body wherein an elongated crystalline starting filament is coupled between a pair oflaterally fixed supports (electric chucks) within a vapor phase deposition zone and maintained at an elevated temperature therein, and the lateral stresses within said filament are relieved by melting a coupling segment between said filament and at least one of said supports prior to the vapor phase deposition. The melted segment will deform to relieve any lateral stresses within the filament. The melted segment can either comprise a small section of'the starting filament, or a fusable portion of a conductor connecting the filament to a support.

In one embodiment of this invention, a segment of the semiconductor filament is melted by the action of an RF coil positioned closely around the filament.

In another embodiment of this invention, a small section of the filament is reduced substantially in diameter so that when current passes through the filament, the section will heat to a temperature higher than the rest of the filament and melt to alleviate the lateral stresses in the filament.

In a further embodiment of this invention. a short, very sharp-pointed section of semiconductor material is positioned in a first electrode chuck and aligned with the filament extending from a second electrode chuck. The pointed segment is then moved to contact the extending end of the filament, and current is passed through the electrode, which action causes the sharp-pointed end to melt and deform to relieve any lateral stresses in the filament.

According to still a further embodiment of this invention, the lateral stresses in a semiconductor filament are relieved'by the melting of a fusable dissimilar metal contained within a coupling unitjoining the filament to an electrode.

It is noted that one purpose of this invention is to reduce the number of crystal dislocations formed during heatup and vapor phase growth of single-crystal material used in semiconductor devices, which dislocations are a known source of degradation of such devices.

This invention can be more easily understood from a study of the drawings in which:

FIG. 1 is an elevational view partly in section showing an embodiment of this invention attached to a reactor for producing a crystalline semiconductor rod by vapor phase deposition;

FIG. 2 is a partial view showing another embodiment'of this invention;

FIG. 3 is a partial view showing still another embodiment of this invention; and

FIG. 4 is a partial view showing an alternate electrode chuck which can be used in the scope of this invention.

Now referring to FIG. 1, reactor 10 comprises a cylindrical quartz reactor tube which is held between end plates 12 and 13 by ring clamps l4 and 15, respectively. End plates 12 and 13 are secured to ring clamps l4 and 15 by nut and bolt assemblies l6. Semiconductor filament 17 is positioned within reactor 10 and held in electrical communication between graphite chucks 18 and 19. Electrode 19a extends through end plate 13 and connects to graphite chuck 19. Electrical coupling device 20 connects between graphite chuck l8 and electrode clamp 21, carrying lead 22. Electrode 19a and lead 22 connect to a conventional electrical power source.

Conduit 23 extends through end plate 13 and serves to introduce reactant gases to the interior of reactor 10. Conduit 24 extends through end plate 12 and functions to remove byproduct gases and unreacted reactants from the interior of reactor 10.

(TI coupling device 20 comprises the improved electrical conducting device disclosed in copending application Ser. No. 783,376 (,I 3364) filed Dec. 12, 1968. Electrical coupling device 20 functions to relieve longitudinal stresses on filament 17 during the heatup and cooldown operation. An alternative electrical coupling device is disclosed in my copending application Ser. No. 744,028, (TI 2904) filed on July ll, 1968. While electrical coupling device 20 is not absolutely necessary when proceeding the alleviate lateral stresses in filament 17 in accordance with the broadest concept of this invention, it is preferred that electrical coupling device 20 be utilized in some embodiments of this invention, to yield a rod having substantially no lateral or longitudinal stress induced therein during the vapor deposition process. it

mm nnnn Electrical coupling device generally comprises a bottom section and a top section which is movable in a vertical relationship to the bottom section. The top section of electrical coupling device 20 comprises gas shield 25, chuck 18, and rod 26 which extends downwardly from the inner face of gas shield into socket chamber 28 of the bottom section of coupling device 20.

The bottom section of coupling device 20 is carried by end plate 12 and generally comprises a tubular housing 27 which encloses cooling chamber 29 and a socket member 28 which is adapted to receive rod 26. The bottom of socket chamber 28 is closed by plug 30.

Plug 30 is generally a hollow cylindrical plug which is closed at its lower end and threaded at its upper end and thereby adapted to threadably engage screw threads positioned in the lower end socket chamber 28. Plug 30 functions to retain a conductive fluid such as a molten or liquid metal, for example, mercury and preferably gallium. Heating device 32 is attached to the bottom of plug 30 for the purpose of supplying sufficient heat to maintain metal 31 in the liquid state. Leads 32a are connected to a conventional power source. Electrode clamp 21 is positioned around plug 30 as described above.

Porous bushing 33, which is generally a gas-permeable cylindrical member, is positioned adjacent the open end socket chamber 28. Bushing 33 can be any tubular porous metal known in the art, such as a porous bronze lubricated bushing which has been heated to remove the oil therefrom. Porous bushing 33 is suspended in the open end of socket 28 by holding members 34 which engage and seat with indentations around either end of bushing 33 to yield an annular space 35 between the wall of socket chamber 28 and the outside periphery of porous bushing 33.

Control gas inlet conduit 36 extends through housing 27,

cooling chamber 20 and the upper portion socket chamber 28 to communicate with annular space 35. Control valve 37 is operatively positioned within control gas inlet conduit 36. Control gas outlet conduit 38 communicates between the interior socket chamber 28 below porous bushing 33 and pressure control valve 39. Conduit 40 is positioned within conduit 38 at a point upstream of control valve 39, and is operatively connected to pressure gauge 41. Cooling chamber 29 is connected to coolant inlet and outlet conduits (not shown).

As shown in FIG. 1 RF coil 42 is operatively positioned around the lower portion of filament 17 as it enters graphite chuck 18. RF coil 42 is suspended within the interior of quartz reactor tube 11 by suitable means such as support arm 43. In addition, RF coil 42 is connected to suitable actuation means by electric lead 44 which communicates through the interior of support arm 43, and through end plate 12.

The basic cylindrical reactor utilized in FIG. 1 can be used for the vapor phase deposition of semiconductor materials known in the art such as, for example, silicon, germanium, and compounds of Groups IIIA and VA of the Periodic Table as illustrated on page 8-2 of The Handbook of Chemistry and Physics, Chemical Rubber Company (1964). However, for the purposes of illustration, this invention will be described in relation to the production ofa silicon rod. In operation, a seed filament of silicon 17 is initially retained between chucks 18 and 19. In most conventional operations the reaction chamber is initially evacuated by a vacuum source and current is then passed through filament 17 until filament 17 is heated to an elevated temperature suitable for etching of the surface thereof. Next, etching vapors such as for example, hydrogen and hydrogen chloride, are passed into the reactor through inlet conduit 23 and exhausted therefrom by outlet conduit 24, in a predetermined manner for a predetermined time.

When operating in accordance with the embodiment of this invention as illustrated in FIG. 1, filament 17 is initially positioned between graphite chucks l8 and 19 through RF coil 42. Electrical coupling device 20 is actuated in a manner to be described below.

Electrical coupling device is operated by initially supplying current through leads 32a to heating device 32 to thereby cause gallium 31 to liquefy. When heating device 32 has melted gallium 31 and rod 26 is free to move within the pool of liquid metal, valves 37 and 39 are opened to allow control gas to pass through porous bushing 33 and out both from under gas shield 25 and conduit 38. A suitable control gas can be any gas which is nondeleterious to the etching procedure and noncontaminating to the subsequent deposition procedure. Preferably, the control gas is hydrogen. When pressure valve 39 is closed, an increased pressure on liquid gallium 31 results, which in turn will force rod 26 upward.

Next, current is passed through filament 17 via electrodes 18 and 19 to thereby cause filament 17 to heat to a temperature at which there is no dislocation generation within filament 17 (below about 960 C.). This heating of filament 17 will cause it to expand in a conventional manner. Next. RF coil 42 is actuated to thereby intensely heat the small segment 17a of filament 17, which is positioned therewithin. This intense heating causes segment 17a to begin to melt. Pressure valve 39 is closed slightly to cause chuck 18 to move upward relative to filament 17, and slightly bulge segment 17a. Any lateral stresses within filament 17 which basically result from angular or positional misalignments of chucks 18 and 19 are achieved by deformation of melted silicon 17a of filament 17.

The longitudinal stresses are next relived prior to etching. Valve 39 is adjusted until the pressure read by gauge 41 will be sufficient to support the weight of the upper section of electrical coupling device 20 comprising rod 26, gas shield 25 and chuck 18, and one-half the weight of filament 17 while providing a sufficient flow of gas to hold rod 26 in a spaced relationship from the inside surface of porous bushing 30. This offsetting pressure will allow filament 17 to expand uniformly during heating without unnecessary distortive forces acting thereon. During this operation, cooling water can be passed through cooling chamber 29.

After the stresses are relieved from filament 17, the vapor phase etching and deposition processes are carried out in a conventional manner. For example, filament 17 is initially heated to a temperature of about 1200 C. and a mixture of hydrogen and hydrogen chloride (about 10 weight percent hydrogen chloride in the hydrogen) is passed through the interior of reactor 10 via conduits 23 and 24 for a period such as one-half hour. Next, suitable reactants are passed to the interior of the reactor such as for example, trichlorosilane. hydrogen chloride and hydrogen to contact filament 17 and deposit pure silicon on the surface thereof.

FIG. 2 illustrates another manner in producing the lateral stress relief according to this invention. As illustrated, the lower portion of filament 17 directly above the portion extending within chuck 18 is ground to a reduced diameter segment 17b, as illustrated. It is noted that the particular position of segment 1712 on filament 17 is not critical, but preferably is located adjacent a chuck. Thus, as current is passed through filament 17 via electrodes 18 and 19 in a conventional manner, the reduced diameter segment 17b will heat much more rapidly than the remaining portion offilament 17, until it melts. The melting will allow distortion of the melted area to thereby offset any lateral stresses within the filament 17. The relative diameter of segment 17b as compared to filament 17 will vary according to the particular situation. When producing a single-crystal silicon body, the relative diameters between section 17b and filament 17 should be sufficient to allow segment 17b to melt before the body of filament 17a reaches dislocation producing temperature of about 960 C. It is noted that during the procedure ofmelting segment 17b, it is preferred that the temperature differential between the larger diameter portion of filament 17 and segment 17b be no less than about 475 C.

Segment 17b can be formed on filament 17 by conventional milling or grinding operations. Since it is very difficult to grind or mill single-crystal silicon to a diameter much smaller than about l/l6 inch before breakage occurs, it has been found when utilizing a relatively small diameter seed filament 17, that an alternate procedure can be utilized to obtain the desired diameter of segment 1712. According to this alternative procedure, segment 17b is initially milled to about 1/16 inch. Then filament 17 is placed between graphite chucks l8 and 19 and heated to a suitable etching temperature, e.g.. about 900 C. This operation will cause segment 17b to become considerably hotter than the body of filament l7 and to thereby etch much faster than the body of filament 17 during the subsequent etching step. Next, etching vapors comprising hydrogen containing about 10 weight percent I-ICl are passed in contact with filament 17. The vapors will etch the entire surface of filament 17, but the action of the vapors on segment 17b will substantially reduce the diameter thereof until the current flowing therethrough causes softening thereof and subsequent release oflateral stresses in filament 17.

After the etching step, the deposition step is carried out in the conventional manner. However, since segment 17b is at an increased temperature, the deposition of silicon occurs at an increased rate thereon until segment 17b reaches a diameter substantially the same as the diameter of filament 17. As the diameter of segment 17b increases, the temperature thereof will correspondingly drop. In this manner, as deposition proceeds, a silicon body of uniform diameter will thereby be formed.

If desired, the heating of segment 1712 can be supplemented by the use of an RF coil 42, as illustrated in FIG. 1, or one or more spot heaters which focus on section 171;. Suitable spot heaters include heaters consisting of a quartz protected tungsten filament and an elliptically shaped reflector.

Now referring to FIG. 3, a preferred embodiment of this invention is illustrated. In this preferred embodiment silicon segment 45 is initially formed which comprises a shank portion 45a adapted to fit within chuck l8 and a pointed portion 45b. Pointed portion 45b is sharpened by a suitable grinding or milling operation to a point having a breadth of from about 1 micron to about 1/32 inch in diameter. Seed filament 17 is next suspended from graphite chuck l9 and then chuck 18 carrying segment 45 is moved upward by the action of electrical coupling device 20 until the tip of pointed portion 45b is in contact with the lower end of seed filament 17 as illustrated in FIG. 0.

Current is passed through filament l7 and segment 45 thereby heating the tip of pointed portion 45b and causing softening and melting thereof. This melting will allow deformation to relieve any stresses in filament 17. It is noted also that the melting of the tip of pointed portion 45b will create molten silicon which will spread over the surface of the end of seed filament 17. This action will create surface tension between the melted silicon and the end of filament 17 to aid in holding filament l7 and segment 45 in intimate contact during the etching and deposition operation.

Now referring to FIG. 4, a further embodiment of this invention is illustrated. Composite coupling member 46 generally comprises a graphite chuck member 47 for receiving the lower end of seed filament 17 and the upper end of connecting member 48. Connecting member 48 has a lower shank portion adapted to fit within chuck l8 and an upper portion having fusable conductor 49 extending concentrically therefrom. Fusable conductor 49 carries graphite extension 50 on the upper end thereof and is adapted to fit within the lower chuck of graphite chuck member 47. Fusable conductor 49 can be made of any suitable metal, such as tin or a tin alloy. Fusable conductor 49 can be either cylindrical or tapered in shape. Graphite chuck member 47 carries a downwardly extending shield 51 positioned concentrically around the lower portion thereof for deflecting reactant gases and decomposition products from the fusable conductor 49. In addition, shield 51 will direct any contaminating vapors emitted from fusable conductor 49 toward outlet conduit 24. It is noted also, that connecting member 48 carries receptacle 52 in the upper end thereof. Receptacle 52 will retain any molten material from fusable conductor 49 during the melting operation.

In the operation of the apparatus as illustrated in FIG. 4, composite coupling member 46 is connected to graphite chuck l8, and seed filament 17 is positioned between graphite chuck l9 and composite coupling member 46. Next, current is passed through filament 17, causing fusable conductor 49 to deform and thereby relieve lateral stresses within seed filament I7. Fusable conductor 49 will melt and cause mating between the lower portion 47a of graphite chuck member 47 and the upper portion 48a of coupling member 48 to thereby allow electrical contact therebetween during the etching and deposition operations.

The following example is given to illustrate a preferred embodiment of this invention, and it is not intended to limit the scope thereof."

EXAMPLE Using a reactor as illustrated in FIG. 1, except that RF coil 42 was not present, an '8 X A inch single-crystal silicon filament was suspended from graphite chuck 19. Next, a silicon segment approximately A inch in diameter and 1 inch long, having a cylindrical check portion and a sharply pointed end portion was positioned in graphite chuck 18 so that the point thereof was directed toward the lower end of the filament suspended from chuck 19. Valves 37 and 39 were actuated to allow electrical coupling device 20 to move upward and the point of the chip to contact the lower end of the seed filament. The reactor was flushed and current was passed through electrodes 18 and 19. Hydrogen was introduced through inlet conduit 23 and exhausted through outlet circuit 24. Pressure valve 39 was regulated so that electrical coupling device 20 supported the weight of rod 26, shield 25, chuck l8, and the pointed segment, and one-half of the weight of the seed filament. The pointed end of the silicon segment melted when the filament reached 600 C. to thereby allow deformation in the coupling segment and the resultant release of lateral stress within seed filament 17. The temperature of the filament was next increased to 1,1 10 C. and maintained at that temperature for 15 minutes. after which the filament was cooled and removed from the reactor. Next, the filament was cut into V2- inch sections and analyzed for dislocations according to the technique disclosed in E. Sirtl and A. Adler in Z. F. Metallk, Volume 52, Number 8, page 529 (1961). No dislocations were found in any of the segments.

It is noted that even though the above example was directed to relieving stresses in a single-crystal starting filament. it must be understood that this invention can also be utilized for relieving stressing in a polycrystalline filament to prevent warpage and breakage thereof.

Thus, while this invention has been described in relation to its preferred embodiments it is to be understood that various modifications thereof will be apparent to one skilled in the art upon reading this specification, and it is intended to cover such modifications as will fall within the scope of the appended claims.

I claim:

1. In a method of growing a crystalline semiconductor body by depositing semiconductor material from a gaseous mixture onto an elongated crystalline starting element coupled between a pair of laterally fixed supports within a vapor deposition zone and maintained at an elevated temperature therein, the improvement comprising:

allowing substantially friction-free lateral movement of said filament by melting a coupling segment between said filament and at least one of said supports to relieve lateral stresses as said filament is heated to said elevated temperature.

2. The method of claim 1 wherein said coupling segment comprises a segment ofsaid starting filament having a reduced diameter.

3. The method of claim 2 wherein said melting is caused by passing electric current through said filament.

4. The method of claim 1 wherein said coupling segment comprises a narrowed portion of semiconductor material held in contact with one of said filament.

5. The method of claim 4 wherein said melting is caused by passing electric current through said filament.

6. The method of claim 1 wherein said coupling segment comprises a fusable metal.

7. The method of claim 6 wherein said melting is caused by passing electric current through said filament.

8. The method of claim 6 wherein said fusable metal is selected from tin and tin alloys.

9. The method of claim 1 wherein said coupling segment comprises a portion of said starting filament which is heated to its melting temperature by the action of an RF coil.

10. The method of claim 1 wherein said melting is caused by heating said coupling segment with an RF coil.

11. The method of claim 1 wherein said starting filament is silicon.

12. An apparatus for producing a body of semiconductor material by vapor phase deposition comprising:

a. an enclosed reaction chamber;

b. inlet conduit means for introducing vaporous reactants to the interior of said enclosed reaction chamber;

c. outlet conduit means for conducting vapors from the interior of said enclosed reaction chamber;

d. a pair spaced-apart electrode chuck means for holding an elongated semiconductor filament within said enclosed reaction chamber;

14. The apparatus of claim 12 wherein said coupling means comprises a heater adapted to intensely heat a small section of said filament.

15. The apparatus of claim 14 wherein said heater comprises an RF coil positioned around said filament.

16. The apparatus of claim 12 wherein said coupling means comprises a pointed semiconductor segment extending from one of said electrode chuck means and adapted to contact one end of said filament extending from the other of said electrode chuck means.

17. The apparatus of claim 12 wherein said coupling means comprises a composite chuck member adapted to seat within one of said'electrode chuck members and extend therefrom to receive one end of said filament, and having a fusable midportion adapted to deform when electric current is passed therethrough. 

2. The method of claim 1 wherein said coupling segment comprises a segment of said starting filament having a reduced diameter.
 3. The method of claim 2 wherein said melting is caused by passing electric current through said filament.
 4. The method of claim 1 wherein said coupling segment comprises a narrowed portion of semiconductor material held in contact with one of said filament.
 5. The method of claim 4 wherein said melting is caused by passing electric current through said filament.
 6. The method of claim 1 wherein said coupling segment comprises a fusable metal.
 7. The method of claim 6 wherein said melting is caused by passing electric current through said filament.
 8. The method of claim 6 wherein said fusable metal is selected from tin and tin alloys.
 9. The method of claim 1 wherein said coupling segment comprises a portion of said starting filament which is heated to its melting temperature by the action of an RF coil.
 10. The method of claim 1 wherein said melting is caused by heating said coupling segment with an RF coil.
 11. The method of claim 1 wherein said starting filament is silicon.
 12. An apparatus for producing a body of semiconductor material by vapor phase deposition comprising: a. an enclosed reaction chamber; b. inlet conduit means for introducing vaporous reactants to the interior of said enclosed reaction chamber; C. outlet conduit means for conducting vapors from the interior of said enclosed reaction chamber; d. a pair spaced-apart electrode chuck means for holding an elongated semiconductor filament within said enclosed reaction chamber; e. means to pass electric current through said electrode chuck means; and f. coupling means adapted to couple said filament with at least one of said electrode chucks comprising a fusable member to laterally deform upon heating to release any lateral stress in said filament.
 13. The apparatus of claim 12 further comprising means to relieve longitudinal stresses in said filament as it is heated.
 14. The apparatus of claim 12 wherein said coupling means comprises a heater adapted to intensely heat a small section of said filament.
 15. The apparatus of claim 14 wherein said heater comprises an RF coil positioned around said filament.
 16. The apparatus of claim 12 wherein said coupling means comprises a pointed semiconductor segment extending from one of said electrode chuck means and adapted to contact one end of said filament extending from the other of said electrode chuck means.
 17. The apparatus of claim 12 wherein said coupling means comprises a composite chuck member adapted to seat within one of said electrode chuck members and extend therefrom to receive one end of said filament, and having a fusable midportion adapted to deform when electric current is passed therethrough. 